Gas shielded arc welding using mixed gases of argon and carbon dioxide provides weld metal containing less oxygen and, therefore, having higher toughness at low temperatures than that obtained from gas shielded arc welding using only carbon dioxide. Furthermore, it reduces spatter, offers high efficiency as it can be used with high currents, and provides excellently shaped beads and good weld appearance. Recently shielding gases containing approximately 20 percent carbon dioxide and a few percent of oxygen in addition to argon have been in wide use.
Flux cored wires comprising steel electrode having a flux core of slag consisting mainly of TiO.sub.2 have been finding increasing use in welding steel structures of complicated shapes, such as offshore structures and ships, as flux cored wires of this type permit welding in vertical and overhead positions. They also form weld metals of good quality and provide high welding performance when used with mixed gases comprising argon and carbon dioxide.
The composition and cross-sectional shape of flux cored wires have great influence on arc stability and welding performance. Because flux consisting mainly of slag is at the core of flux cored wires, arcs produced by them tend to spread more than those produced by solid wires. Widespread arcs have an advantage that convex beads are less likely to be formed by downhand and horizontal fillet welding. On the other hand, they tend to form disfigured weld beads in vertical and overhead welding as they are less focused.
Particularly when vertical position welding is done using wires having flux cores of nonuniform segregated compositions with high welding currents of over 250 A, arcs and beads become unstable and the incidence of slag inclusions and other weld defects increases. Therefore, people doing semiautomatic welding have been required to have great skills in the manipulation of electrode wires and the maintenance of uniform arc length.
As the use of automatic welding increases, automatically and semiautomatically welded parts have come to be mixed in more welded structures than before. When automatically welded beads formed by using solid wires and semiautomatically welded beads formed by using conventional flux cored wires are compared, the latter are less properly shaped and more likely to form uneven toes. Therefore, flux cored wires for semiautomatic welding have been required to be improved to produce beads and toes more analogous to those formed by automatic welding.
Japanese Provisional Patent Publication No. 193293 of 1991 proposes flux cored wires with improved arc stability in which the ratio of metal powder to flux in their cross-sectional area is 10 to 90%. Japanese Provisional Patent Publication No. 80683 of 1995 proposes wires in which iron powder is placed at the center of the their cross section. Both wires prevent spatter of weld metal and stabilize the arc by reducing the protrusion of unmolten flux. Still higher arc stability is required for the attainment of better bead shapes.